This invention relates to multi format display systems for cathode ray tubes (CRTs). A conventional CRT display has an aspect ratio (AR), i.e. the ratio of its horizontal and vertical dimensions, of 4:3. Recently proposed high definition television systems include CRTs having 16:9 aspect ratios to provide displays that are more like those used in the movies. A difficulty arises in that the phosphor screen of a 16:9 aspect ratio CRT is only partially illuminated when displaying a 4:3 AR video signal. A number of uses for the unused portions of the CRT have been proposed. U.S. Pat. No. 5,159,438, entitled, ASPECT RATIO CONVERSION OF TELEVISION DISPLAY, in the name of K. Rabii and assigned to Zenith Electronics Corporation, discloses a technique for converting a 4:3 AR video signal to a 16:9 format to permit use of the entire CRT surface. Some proposals simply cover the so-called "side panels," or unilluminated portions of the CRT that result, with a movable curtain or mask.
The present invention is directed to an anticipated problem in multi format display systems due to phosphor aging of the CRT screen. All phosphors are subject to aging (browning), i.e. their light output diminishes with use. A 16:9 AR CRT that is also used for displaying 4:3 AR video will have significant portions of its phosphor display surface that are not activated in the 4:3 display mode. Thus portions of the CRT phosphor screen will experience different levels of usage. After some period of operation, the phosphor surface that experiences less use will be brighter (for the same beam current) than the phosphor surface that is used more often. The result will be a difference in brightness across the face of the CRT, which could prove most distracting when viewing a high definition television display.
The present invention minimizes the possibility of such an eventuality by providing low level illumination for the unused portions of the CRT phosphor screen when operating with 4:3 AR video displays.